<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    var month = parseInt(prompt('请输入月份', '2'));
    var year = parseInt(prompt('请输入年份', '2020'));
    switch (month) {
      case 1:
      case 3:
      case 5:
      case 7:
      case 8:
      case 10:
      case 12:
        alert(year + '年的' + month + "月有31天");
        break;
      case 4:
      case 6:
      case 9:
      case 11:
        alert(year + '年的' + month + "月有30天");
        break;
      case 2:
        //2月要考虑闰年 
        if (year % 4 === 0 && year % 100 !== 0 || year % 400 === 0) {
          alert(year + '年的' + month + "月有29天");
        } else {
          alert(year + '年的' + month + "月有28天");
        }
    }
  </script>
</body>

</html>